1. Field of the Invention
The present invention relates generally to a driver circuit for driving a piezoelectric element, and more particularly to such a driver circuit which is economical to manufacture and which requires reduced electric power consumption and generates reduced amount of heat.
2. Discussion of the Prior Art
A piezoelectric element oscillates at a given frequency, with alternate repetition of charging and discharging. In some applications of the piezoelectric element as an actuator, it is desirable that the piezoelectric element be held in the same displaced state for more than a predetermined time duration.
For example, the above desire exists when the piezoelectric element is used as an actuator for operating a print wire of a print head of a dot-matrix impact printer.
The present applicant et al have proposed a driver circuit for driving a piezoelectric element, as disclosed in co-pending application Ser. No. 07/426,773 filed Oct. 26, 1989. The driver circuit disclosed therein includes a charging circuit having a DC power source and a coil which are connected in series to the piezoelectric element for charging the piezoelectric element, and a discharging circuit for allowing the piezoelectric element to be discharged. The driver circuit comprises: (a) first conditioning means provided in the charging circuit, and having an original state for inhibiting the piezoelectric element from being charged, and a charging state for allowing the piezoelectric element to be charged, the first conditioning means being normally placed in the original state, and being brought into the charging state upon generation of a drive command to activate the piezoelectric element, the first conditioning means being returned to the original state at a first point of time a predetermined time after a voltage of the piezoelectric element has reached a predetermined level not higher than a line voltage of the power source; (b) second conditioning means provided in the discharging circuit, and having an original state for inhibiting the piezoelectric element from being discharged, and a discharging state for allowing the piezoelectric element to be discharged, the second conditioning means being normally placed in the original state, and being brought into the discharging state thereof at the first point of time indicated above, or at a second point of time a predetermined time after the first point, the second conditioning means being returned to the original state by the time when the first conditioning means is placed in the charging state for the next activation of the piezoelectric element; and (c) voltage limiting means for preventing the piezoelectric element from receiving an excessive electric energy that causes the voltage of the piezoelectric element to exceed the predetermined level indicated above, and thereby limiting the voltage of the element to the predetermined level.
In the driver circuit constructed as described above, the voltage limiting means (in the form of diodes, for example) permits an electric current to flow in a direction from the coil toward the power source after the voltage of the piezoelectric element has reached the predetermined level during charging of the element. Consequently, the electric energy is stored in the coil, preventing the voltage of the piezoelectric element from exceeding the line voltage of the power source, thereby maintaining the piezoelectric element at the predetermined level, after the predetermined level has been reached.
The application of the drive voltage to charge the piezoelectric element is started in response to the drive command, and continues for a suitable period, which expires when or after the voltage of the piezoelectric element has reached the predetermined level.
After the charging time has expired, the electric energy stored in the coil is returned to the DC power source for some period, during which the piezoelectric element cannot be discharged. In other words, the discharging of the piezoelectric element tends to be delayed, causing a relatively long time between the moment of termination of the current charging cycle and the moment of commencement of the next charging cycle. Consequently, the required cycle time including the charging and discharging times tends to be relatively long, causing an accordingly low operating speed of the piezoelectric actuator.
Another drawback of the driver circuit discussed above is a comparatively large amount of electric power consumption due to the electric current flow through the coil for the relatively long charging time with the drive voltage kept applied to the piezoelectric element until the voltage of the piezoelectric element has reached the predetermined level.
As a result of a continuing study in an effort to solve the above drawbacks, the applicant found that the voltage of the piezoelectric element will rise up to the line voltage of the DC power source, even if the application of the drive voltage is terminated before the voltage of the piezoelectric element has reached the predetermined level not higher than the line voltage.